releasetools: Pass --virtual-ab to lpmake.

Note that we are currently excluding this flag for retrofit devices,
since it necessitates support for a newer super.img format, which
retrofit devices may not fully support.

Bug: 134949511
Test: m -j, lpdump super_empty.img
Change-Id: Id69a8d755b9f4c4bb5fbd456a416ced1a5f1d135